Virtual machine extension (VMX) architecture allows multiple software programs and operating systems to use the same microprocessor logic (“hardware”) by allocating processor resources to various software applications and operating systems at different times. One feature of the VMX architecture allows software programs (“guest software”) to be executed one instruction at a time. Executing instructions of a software program one at a time is typically used as a debug technique and may be referred to as “single stepping” the program.
The VMX architecture typically uses a virtual machine monitor (VMM) program that interfaces one or more software programs, such as a virtual machine (VM), to a single microprocessor or collection of processing elements. The guest software running on each VM may include a guest operating system and various guest software applications. Furthermore, an application and operating system running on a VMM may be collectively called a VM or a guest. FIG. 1 illustrates a prior art structure (“virtual machine environment”) for interfacing guest software to a microprocessor. Specifically, FIG. 1 illustrates a VMM that interface two VMs (“guest software”) to a microprocessor. The software running within each VM may include a guest operating system as well as various software applications. In order to interface each VM to processor resources, such as registers, memory, and input/output (“I/O”) resources, state and control information is modified and otherwise tracked via a set of fields within a virtual machine control structure (VMCS). More specifically, a control structure, such as the VMCS, is typically used to pass control of and access to processor resources between the VMM and a VM guest.
One feature of the VMX architecture is the ability to single step through guest software. In order to single step through guest software, the VMM typically, among other things, sets a control field or bit, such as a called Monitor Trap Flag (MTF), in the VMCS.
After the MTF bit is set in the VMCS, the VMM may request a transfer of access and/or control of processor resources to the guest software. The VMM may request access to and/or control of processing resources by issuing an instruction, such as a VMLAUNCH or VMRESUME instruction. A VMLAUNCH instruction is typically executed in order to execute a guest application for the first time. Thereafter, a VMRESUME instruction may be used to perform the same function.
Prior art techniques of single stepping through a guest software routine or routines can often be susceptible failure if an event, such as an exception, occurs during the execution of the guest software. Typically, successful execution of instructions within a guest software context yields control to the VMM whenever a VM exit event occurs. However, some VMM instructions, such as VMLAUNCH and VMRESUME, which transfer control to guest, do not.